/Engine/Source/Runtime/Core/Public/UObject/WeakObjectPtrTemplates.h
Slightly different than !IsValid(), returns true if this used to point to aUObject, but doesn't any more and has not been assigned or reset in the mean time.
true if this used to point at a real object but no longer does.
bool IsStale ( bool bIncludingIfPendingKill, bool bThreadsafeTest ) const